1. Field of the Invention
The present invention relates to a magnetic recording medium using a ferromagnetic metal thin film and in particular, to a magnetic recording medium which can preferably be used as a video tape for a long-time recording or a tape streamer of a large capacity.
2. Description of the Prior Art
Conventionally, polyethylene terephthalate has been widely used as a substrate forametal thin film type magnetic recording medium. Home video cassette tapes such as an 8 mm tape use a polyethylene terephthalate film having a thickness of 7 to 10 micrometers. Tape streamers for computer data backup use a polyethylene terephthalate film having a thickness of 5 to 7 micrometers.
Recently, with reduction in the video cassette size, there is a need for a video cassette magnetic recording medium reduced in size and capable of recording for a long time. Moreover, with increased of the information amount, tape streamers are also required to have a larger capacity. In order to satisfy these requirements, there has been examined to reduce the thickness of the magnetic recording medium.
To reduce a thickness of a magnetic recording medium, one approach is to reduce the thickness of the substrate. However, reduction of the substrate thickness brings about lowering of stiffness of the magnetic recording medium and deterioration of the skew characteristic.
To reduce the substrate thickness while satisfying the stiffness and the skew characteristic of the magnetic recording medium, it is necessary to obtain a substrate which simultaneously has a high strength and a low heat absorption which oppose to each other.
When using a polyethylene terephthalate film as a substrate, a technique such as a repeated expansion has been used to satisfy the aforementioned requirement, or ageing has been applied to reduce heat absorption.
However, the thickness of substrate is still being reduced. There is already a limit in the strength characteristic when using the polyethylene terephthalate.
To cope with this, a polyamide film has been examined and began to be used in practice as a substrate of the metal thin film type magnetic recording medium.
A polyamide film has a higher strength than the polyethylene terephthalate film Accordingly, a magnetic recording medium using this polyamide film as the substrate can be reduced in thickness. A great expectation is posed on this polyamide film for a magnetic recording medium to answer to the increased recording time of the video cassette and the larger capacity of the tape streamer.
In general, to increase the recording density of a magnetic recording medium, it is necessary to obtain a flat and smooth surface and to reduce a spacing loss as much as possible. However, if a magnetic recording medium has a surface too flat and smooth causes a problem in the head touch and running characteristics. Accordingly, it is necessary to control the fine surface configuration to assure an appropriate surface characteristic.
Furthermore, the relative speed between a magnetic head and a magnetic recording medium has been increased. There is a need to obtain a magnetic recording medium having a high durability with little wearing in the magnetic layer. From this viewpoint also, it is indispensable to control the fine surface configuration of the magnetic recording medium.
On the other hand, a metal thin film type magnetic recording medium has a magnetic layer made from a ferromagnetic metal thin film which is very thin. Accordingly, the magnetic layer is easily affected by the surface of the substrate. That is, the surface state of the non-magnetic substrate, i.e., surface roughness directly appears as the roughness of the magnetic layer surface.
Consequently, in the metal thin film type magnetic recording medium it is necessary to control the surface configuration of the non-magnetic substrate so as to obtain an appropriate state of the fine configuration of the magnetic layer surface.